Vulcanization of rubber



Patented Apr. 16, 1935 UNITED STATES PATENT OFFICE.

VULCANIZATION F RUBBER Ira Williams, Woodstown, N. J., assignor to E. I.du-Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application March 25, 1933,

Serial No. 662,825

4 Claims.

5 cilitated. o

It is well known that rubber which is deficient in fatty acid contentproduces a poor vulcanizate and that rubber from which all acidicmaterial has been removed cannot be satisfactorily vulcanized. It isalso known that many accelerators of vulcanization require the additionof considerable fatty acid in order to produce satisfactoryacceleration. Accordingly, it has become a practice to add fatty acidsof the higher fatty acid series to rubber mixes. o

The fatty acids most commonly employed are stearic acid and oleic acid.The use of these and other high molecular weight fatty acids aredisclosed in Patent 1,467,197, granted to W. F. Russell. While theseacids will overcome the fatty acid deficiencies of rubber, they areobjectionable in several respects. the presence of such accelerators astetramethyl thiuram monosulfide or butyraldehyde-aniline condensationproducts, they have a retarding action on the vulcanization due'to theiracidity. When employed in amounts greater than about 1%, these materialsbloom to the surface and destroy the tack of the unvulcanized rubber insuch manner as to seriously interfere with fabricating operations.

An object of the present invention is to provide a class of materialswhich may be employed to correct the acid deficiency of rubber and atthe same time to act as accelerator activators without exerting aretarding action on certain accelerators which are ordinarilydeleteriously affected by fatty acids. A further object is to provide aclass of acid supplying accelerator activators which are sufficientlysoluble in rubber to permit them to be employed in the desired amountswithout blooming to the surface. Still further objects are to improvethe properties of rubber and to provide methods for effecting suchimprovement. Other objects are to provide new compositions of matter andto advance the art. Other objects will appear hereinafter.

These objects may be accomplished in accordance with my invention whichcomprises adding to a rubber mix, prior to vulcanization, certain Whenemployed in mono-carboxylic aliphatic acids, or their salts andthereafter vulcanizing. The mono-carboxylic acids and their salts whichare to be employed in accordance with my invention comprise compounds ofthe type:

in which M represents hydrogen or an equivalent of a metal selected fromthe group comprising 1 lead, mercury, cadmium and zinc and R representsa'saturated or unsaturated alkyl group of more than one and less than 10carbon atoms and in which the longest carbon chain is not more than 7carbon atoms in length, or R. may represent a saturated cyclichydrocarbon group.

Among the acids which have been found to be particularly satisfactoryfor my purpose are propionic, butyric, isobutyric, crotonic, valeric,caproic, heptoic, ethyl propylacetic, 3-methyl-5- ethyl-hexoic, andhexahydrobenzoic acids, and their salts of lead, mercury, cadmium andzinc.

These acids are preferably employed by first forming the zinc salt andadding such salt to the rubber mix. However, if desired, these acids maybe introduced into the rubber in the form of the free acid together withexcess of the metallic oxide whereupon the metallic salt will be formedin the rubber. The use of the acid in the form of its salts willgenerally be preferred to avoid disadvantages attendant upon adding amaterial which may be volatile and of disagreeable odor.

In order to determine the effectiveness of this class of materials, arubber mix was prepared 0 having the following composition:

Parts by weight Rubber, 100 Carbon black Zinc oxide 5 Sulfur 3Mercaptobenzo-thiazole .5

some of these tests are shown in the following table:

compounds in the class may be employed with satisfactory results, theproportions may be varied Tensile strength at break Activator addedMinutes cure None Zinc pro- Zinc iso- Zinc hep- Crotonic Cadmium Leadbuty- Mercurous Oaproic pionate butyrate toate acid propionate ratecaproate acid The amounts of these materials to be employed will varyaccording to the composition of the rubber and the rubber mix and otherconditions in a manner which will be readily apparent to one skilled inthe art. For example, certain types of rubber may be more deficient infatty acid than others and hence will require the use of larger amountsof material. Generally speaking, relatively small amounts are sufficientto produce the desired result and amounts of from 0.1 to 4.0% willusually be found to be satisfactory in most cases.

It is also well known that certain accelerators, such as diphenylguanidine are affected by the presence of excessive amounts of free acidand in such cases the smallest amount of fatty acid which will overcomethe fatty acid deficiency should be employed or the acids should be usedin the form of their salts such as the zinc and lead salts. I have foundthat, when the acids of my invention are employed, particularly in theform of metallic salts, they will not deleteriously aifect the action ofaccelerators such as tetra-methyl-thiuram monosulfide,butyraldehyde-aniline condensation products and diphenyl guanidine.

While the acids and their salts are primarily intended to be employed inconjunction with organic accelerators, advantageous results may beobtained by employing these materials in the absence of added organicaccelerator.

The acids of my invention and their metallic salts are sufiicientlysoluble in rubber so that effective amounts may be employed without theproduction of surface bloom or a non-tacky surface. Accordingly, thesecompounds are particularly useful when a tacky surface is required tofacilitate fabricating and building operations.

While I have disclosed the use of specific compounds within the classemployed in specific proportions in only one rubber mix, it will bereadily apparent to those skilled in the art that other and they may beemployed in other types of rubber mixes than that disclosed withoutdeparting from the spirit of my invention. Accordingly, the scope of myinvention is to be limited solely by the appended claims, construed asbroadly as is permissible in view of the prior art.

I claim:

1. The method of vulcanizing rubber which comprises incorporating in arubber mix an organic accelerator of the type represented bymercaptobenzothiazole, diphenyl guanidine, tetramethyl thiurammono-sulfide and butyraldehydeaniline and a small amount of an activatorcomprising a metal salt of propionic acid in which the metal is a memberor the group consisting of zinc, lead, mercury and cadmium, and thensubjccting the mix to a vulcanizing step.

2. The method of vulcanizing rubber which comprises incorporating in arubber mix an organic accelerator of the type representedbymercaptobenzothiazole, diphenyl guanidine, tetramethyl thiurammono-sulfide and butyraldehyde-aniline and a small amount of anactivator comprising zinc propionate, and then subjecting the mix to avulcanizing step.

3. Rubber having incorporated therein an organic accelerator of the typerepresented by mercaptobenzothiazole, diphenyl guam'dine, tetramethylthiuram mono-sulfide and butyraldehydeaniline and a small amount oi anactivator comprising a metal salt of propionic acid in which the metalis a member of the group consisting of zinc, lead, mercury and cadmium.

4. Rubber having incorporated therein an orgariic accelerator of thetype represented by mercaptobenzothiazole, diphenyl guanidine,tetramethyl thiuram mono-sulfide and butyraldehydeaniline and a smallamount of an activator comprising zinc propionate.

IRA WILLIAMS.

